The scale-up of antiretroviral therapy has transformed the health of HIV-endemic populations in South Africa, with unprecedented gains in life expectancy (Bor et al. 2013) and significant reductions in HIV incidence (Tanser et al. 2013). To achieve the full potential benefits of ART scale-up, further evidence is needed on the pathways (behavioral vs. biological) through which ART coverage reduces incidence, and the reasons why so many people with HIV continue to die when life-saving ART is available. We will establish this evidence base through analysis of demographic and health surveillance data from rural KwaZulu-Natal. This research will inform interventions to maximize the impact of South Africa's ART program.